1. Field of the Invention
The invention makes bends of small bending radii possible in low-index difference waveguides, and further for efficient coupling from/to fibers and between two vertically positioned waveguides. These optical interconnections enable an increased density of integration of optical components on one chip.
2. Prior Art
Waveguide bends, splitters couplers are essential components of optical circuits. The well-developed xe2x80x9csilica benchxe2x80x9d technology accommodates waveguide interferometric filters for multiplexing and de-multiplexing and other optical components. The refractive index contrast between the waveguide cores and claddings is small, leading to waveguide cross-sections that are relatively large and index matched so as to permit coupling from and to optical fibers with low insertion loss. This is one of the reasons for the wide application of the technology. However, the density of integration of optical components is limited by the fact that, with waveguides formed via small index contrast, bends must possess relatively large bending radii to keep the radiation losses within acceptable bounds.
The index difference xcex94n between the core and cladding is predominantly controlled by impurity doping such as Ge, B, etc., in silica bench technology. Thus, xcex94n is typically controlled to be 0.01 or less, resulting in bends with a curvature on the order of mm. Since a 90xc2x0 splitter of the waveguide consists of two bends, the structure requires as large curvature as bends. Thus, silica bench integrated optical circuits possess around 10xc3x9710cm2 in area.
It is obvious that increase of xcex94n in the bench should lower the curvature of bends and splitters. It is, however, difficult to find cladding materials with refractive indices lower than silica, since the refractive index of silica core is as low as xcx9c1.5. Thus, the silica technology stays in low-density circuitry on a relatively large bench.
In addition to above reasons, even if one adopts high index contrast system in integrated optical circuits, the mode size difference between optical fiber and integrated optical circuits becomes large and Fresnel reflection at the interface between optical fiber and integrated optical circuits brings about huge junction loss.
In order to increase the difference of refractive index between the core and cladding, use of air cladding is effective. Regarding air cladding, U.S. Pat. No. 3,712,705 issued to Marcatili et al. shows it in case of optical fibers. The air-clad optical fiber is described as including a low-loss dielectric core, having a polygonal cross section disposed within a circular jacket. Because of its shape, the core is, in effect, totally surrounded by air, which makes xcex94n larger.
In the invention, waveguide bends having two or more sidewalls interfaced with air trench claddings are provided. The invention uses the principles of index guidance, adiabatic transition, and mode matching to counter Fresnel reflection induced junction loss.
It is an objective of the invention to provide a bend and a splitter of a waveguide with a relatively small curvature, and to provide a coupler from/to fiber and between out-of plane waveguides. It is another objective of the invention to provide a fabrication method for these structures. Wafer bonding allows us to incorporate air as the low-index material.
The invention addresses waveguide bends and crossings of waveguides of low index contrast using local air-trenches at the bends and crossings which increase the index difference locally to xcx9c0.5 or larger, depending upon the core index. In the invention, air is chosen as the low index material as an example, but other materials including low-index polymers and insulators can also be used instead of air for the trenches.